1. Field of the Invention
The present invention relates to a motor control apparatus for controlling a direct current (DC) motor and an image forming apparatus including the motor control apparatus.
2. Description of the Related Art
There are copying machines and printers, in which a toner image is formed on photosensitive drums and transferred to an intermediate transfer belt, and the toner image is transferred from the intermediate transfer belt to paper. To obtain better image quality, those machines are configured such that separate motors are used to drive the photosensitive drums and the intermediate transfer belt. In this case, since the photosensitive drums and the intermediate transfer belt contact with each other at positions of image transfer, if there is a difference between their circumferential speeds when they rotate, their surfaces may get scratched. There is no such problem when the circumferential speeds of the photosensitive drums and the intermediate transfer belt are controlled to a target speed. However, when the photosensitive drums and the intermediate transfer belt are driven by separate motors (DC motors, for example), the motor startup characteristics (e.g., overshoot) from when a motor is started and accelerated until its speed rises up to a target angular speed differ depending on motors. Therefore, so long as the startup characteristics of the motors cannot be made equal, a difference arises in the circumferential speed between the photosensitive drums and the intermediate transfer belt.
It has been proposed to increase a pulse width modulation (PWM) signal, which controls the revolution number, from a low duty ratio at a predetermined increase rate to start a DC motor with little overshoot (Japanese Patent Application Laid-Open No. 1995-039181). In addition, it has been proposed to control a clock frequency which is to be input to a DC motor, to be a frequency lower than a target frequency when the DC motor is started and accelerated, and then increase the clock frequency input to the DC motor to the target frequency (Japanese Patent Application Laid-Open No. 2007-156238).
However, in FIG. 11B, even if the angular speed is controlled to start at a low control value, when a DC motor is started and accelerated, and increase to a target control value, when the angular speed comes closer to the target angular speed, its increase turns to be non-linear, as illustrated in FIG. 11A, though the angular speed of the DC motor increases linearly up to halfway to the target angular speed. It is difficult to match the nonlinear characteristics among the different motors. Therefore, if the angular speeds of DC motors can be started and accelerated linearly up to the target value, startup characteristics of the motors can be brought in line.